The present invention relates to a roll coater with which an interlaminar resin insulating layer or a solder resist layer can be formed with good thickness uniformity and to a method of manufacturing a printed circuit board which utilizes said roll coater.
The multi-layer printed circuit board which is commonly referred to as multilayer built-up printed circuit board has heretofore been manufactured typically by the semi-additive method in which a copper or other conductor circuit and an interlaminar resin insulating layer are alternately built up on a glass cloth- or other reinforced resin substrate about 0.5 to 1.5 mm thick which is generally called xe2x80x9ccorexe2x80x9d. Conductor circuit-circuit interconnection through said interlaminar resin insulating layer of the multilayer printed circuit board is achieved by means of via-hole.
The built-up multilayer printed circuit board is conventionally manufactured by the process described in JP H09-130050 A, for instance.
Thus, holes are drilled through the thickness of a copper-clad laminate board in the first place and electroless copper plating is then carried out to form plated-through holes. Then, the surface of the substrate is etched pattern-wise to form a conductor circuit and the surface of the conductor circuit is roughened by electroless plating or etching. On the conductor circuit pattern having such a roughened surface, a resin insulating layer is constructed and subjected to exposure to light and development to form openings for via-hole. Then, UV curing and postcuring are carried out to complete the interlaminar resin insulating layer.
Further, the surface of interlaminar resin insulating layer is roughened with an acid or an oxidizing agent and an electroless plating layer is formed thereon. After a plating resist is applied onto this electroless plating film, salvage electroplating is carried out, followed by removal of the plating resist and etching to form a conductor circuit which is electrically connected to an under-level conductor circuit through said via-hole.
After the above procedure is repeated, finally a solder resist layer for protecting the conductor circuit is formed, the exposed hole for connection to electronic components, such as an IC chip, a mother board, etc. are plated, and solder bumps are formed by printing with a solder paste to complete the manufacture of a built-up multilayer printed circuit board.
Regarding the method of forming an interlaminar resin insulating layer in the manufacture of a multilayer printed circuit board, JP H10-65348 A discloses a method of manufacturing a printed circuit board which comprises setting a substrate in vertical position between two co-operating rolls of a roll coater to concurrently coat both sides of the substrate with an interlaminar insulating resin coating to form an interlaminar resin insulating layer on either side of the substrate.
This method of manufacturing a printed circuit board in which a coating layer is formed on either side of a substrate in one operation is advantageous not only in terms of productivity but also in that because the interlaminar insulating coating is dried on both sides of the substrate concurrently, the curling of the substrate resulting from drying of the interlaminar insulating coating is precluded.
Meanwhile, JP H10-65347 A discloses a method of manufacturing a printed circuit board which includes a step of applying the interlaminar insulating coating only to the inner area exclusive of both marginal areas of a substrate.
This method of manufacturing a printed circuit board offers the advantage that the substrate can be handled and transferred without incurring contamination of the interlaminar resin insulating layer with dust particles.
However, an interlaminar resin insulating layer, and for that matter a solder resist layer, as formed by using the roll coater described in the above patent literature tends to have the drawback of uneven coating thickness.
Thus, in order to level out the streak pattern that would be produced by said roll coater, the roll coating is performed under pressure application but when the substrate is smaller in width than the coating roll, the edge portions of the substrate are subjected to excessive pressure with the consequent reduction in thickness of the marginal areas. Moreover, when the coating is carried out using a coating roll smaller in width than the substrate from handling considerations, the edges of the rubber member of the roll under pressure expand outwardly in the lateral direction to cause local reductions in pressure so that the thickness of the coat is locally increased.
When a mask drawing circular marks for via-hole or solder pad, or other marks is placed on such an interlaminar resin insulating layer having relatively thickened edges or an uneven thickness, the mask is tilted along the gradient of thickness with the consequence that the ultraviolet or other exposure light from a light source is incident aslant so that holes of the designed geometries cannot be formed.
The failure to form a hole of the designed geometry results in a connection failure or an increased risk for disconnection between the conductor circuit formed on the interlaminar resin insulating layer and the under-level conductor circuit beneath the insulating layer in the formation of via-hole. Moreover, in the heat cycle test and the reliability test under high-temperature, high-humidity condition, the exfoliation and disconnection of the conductor within the via-hole tend to take place.
Furthermore, the morphologic uniformity of the solder bump formed at the apertures of the solder resist layer cannot be obtained, leading to a failure to establish connection with an IC chip and other electronic components and difficulties in the formation of an anticorrosive layer of nickel, palladium or other noble metal on the conductor circuit. In addition, peeling of the anticorrosive metal layer or the solder bump and early onset of corrosion may occur to adversely affect electrical continuity and reliability.
Having been developed to overcome the above disadvantages, the present invention has for its object to provide a roll coater with which the interlaminar resin insulating layer and the solder resist layer can be formed each in a uniform thickness and no-hole defect anomalies in the diameter and geometry of the opening for via-hole or solder bump can be precluded to provide a printed circuit board improved in electrical continuity and operational reliability and a method of manufacturing such a printed circuit board using said roll coater.
As the result of an intensive investigation made to accomplish the above object, the inventors of the present invention found that when the amount of the interlaminar insulating coating to be held by the coating roll in both end marginal areas of its surface is relatively reduced, the coating amount in said marginal areas will not become excessive even if the interlaminar insulating coating applied to the central area of the roll surface is forced out towards said marginal areas so that the resulting interlaminar resin insulating layer will be uniformized. The present invention having the following cardinal feature has been developed on the basis of the above finding.
The roll coater of the present invention is a coating apparatus for use in the formation of an interlaminar resin insulating layer and/or a solder resist layer of a printed circuit board comprising a substrate and, as serially built up thereon, a conductor layer and an interlaminar resin insulating layer in an alternate fashion and in repetition, with a solder resist layer formed on top of the resulting multilayer structure, which roll coater comprising a roll each having a surface formed with a multiplicity of grooves running in the direction of roll rotation, with the depth of the grooves in both marginal (peripheral end) areas of the roll surface being relatively reduced as compared with the depth of the grooves in the remaining area.
Preferably, in the above roll coater, said depth of grooves in said marginal areas are progressively reduced toward the corresponding ends of the roll.
The depth of the grooves in outermost regions of said marginal areas is preferably 10 to 50% less than the depth of the grooves in other region.
Furthermore, the width of each marginal area where the depth of grooves is relatively reduced as compared with the remaining area is preferably not greater than 5 cm.
In addition, both edges of the roll coater are preferably comprising a guide at each end of the roll.
The method of manufacturing a printed circuit board according to the present invention comprises forming a conductor layer and an interlaminar resin insulating layer serially in an alternate fashion and in repetition on a substrate and a solder resist layer on top of the resulting multilayer structure, characterized in that said interlaminar resin insulating layer and/or said solder resist layer is formed by means of the above-described roll coater.